As the exploration for oil and gas expands ever further offshore, it is necessary to moor mobile offshore drilling units (MODU's) and floating production platforms (FPP's) in very deep waters. In many circumstances it is advantageous to moor MODU's and FPP's using preset mooring systems. A preset mooring system typically comprises an array of eight or more anchors each having a mooring line extending therefrom. The array of anchors is installed around a mooring location after which an MODU or a FPP is positioned at the mooring location and is moored utilizing the anchors and the mooring lines of the preset mooring system.
In order to avoid costly delays and reworking operations, the holding power of each anchor in an anchor array of a preset mooring system is carefully tested prior to the connection of an MODU or an FPP to the anchor. Heretofore various techniques have been utilized in anchor testing procedures. These include: use of the anchor installation ship to apply load to a single anchor up to the power limitation (Bollard pull) of the ship; use of the anchor installation ship to apply a minimum setting load to two diametrically opposed anchors in combination with the use of a tensioning barge to apply full test loads to the two anchors; testing a single anchor by connecting a deadman anchor to the bow pad eye of the anchor installation ship and applying load to the anchor using a combination of the ship's Bollard pull and its towing winch pull capacity; and/or using a ratchet tensioning system which utilizes the pulling capacity of the installation ship's towing winch. All of the foregoing techniques are considered to be unsatisfactory for one or more reasons, including excessive costs, limitations as to depth, and limitations as to capacity.
The present invention comprises an anchor installation vessel and method which overcomes the foregoing and other problems long since associated with the prior art. In accordance with the broader aspects of the invention, an anchor installation vessel is adapted to transport and install at least two of the anchors and the mooring lines associated therewith which form part of the anchor array of a preset mooring system. The vessel is provided with a pair of winches and structure for connecting the winches to two diametrically opposed anchors of the anchor array. In this manner the anchor installation vessel of the present invention is adapted to simultaneously test two anchors by employing the anchors to pre-load each other.
In accordance with more particular aspects of the invention, the rear of the vessel is provided with a pair of transverse rollers and a pair of longitudinal rollers located adjacent to and below the transverse rollers. This allows the mooring lines extending from the anchors to enter the vessel from opposite sides. As load is applied to the mooring lines by the winches the stern of the vessel centers itself both transversely and longitudinally at a center point between the two anchors, and the anchors are tested by employing the winches of the vessel to apply equal and opposite loads thereto.